The present invention relates to the class of angular rate sensors wherein two waves propagate or travel in opposite directions in a closed-loop path and which include biasing systems for minimizing the effects of lock-in caused by back scattering. In particular, the present invention provides a novel method of accounting for inherent errors typically included in sensors of this type whereby the sensors may be controlled so as to reduce the magnitude of error, or the sensor output can be corrected or compensated for the error.
In a simple laser angular rate sensor, sometimes referred to as a ring laser gyro, two counter-traveling waves are provided by two waves or beams of substantially monochromatic electromagnetic radiation, usually two monochromatic beams of light. The two light beams are generated so as to travel in opposite directions along a closed-loop path which typically, through not necessarily, encloses the input axis about which rotation is to be sensed. When the sensor is at rest, the lasing path is identical for the oppositely traveling beams resulting in the frequency of oscillation of each beam being identical. Rotation of the ring laser gyro, particularly rotation of the closed-loop path, about the input axis causes the effective lasing path length travelled by one beam to increase, while the effective lasing path length travelled by the other beam to decrease. The resulting change in path length of the two beams produces a frequency change in each of the beams, one increasing and the other decreasing, since the frequency of oscillation of the beam of electromagnetic radiation in such systems is dependent upon the effective length of the lasing path. The frequency difference between the two beams is therefore indicative of rotation rate of the beams of light, i.e. the rotation rate of the closed-loop path about the input axis. A frequency difference between the two beams results in a phase shift between the counter-traveling beams which changes at a rate proportional to the frequency difference. Thus, phase shift between the two beams is proportional to the time integral of the frequency difference, and is representative of the time integral of the input rotation rate about the gyro input axis. The total phase shift over a time interval is, therefore, indicative of the total angular displacement about the gyro input axis during the integrated time interval, and the rate of change of phase shift thereof is indicative of the rate of rotation about the gyro input axis.
A bothersome characteristic of the ring laser gyro is "lock-in". At rotation rates about the input axis of the ring laser gyro below some critical value called the lock-in threshold or lock-in rate, the frequency difference between the oppositely traveling beams synchronize to a common value resulting in the frequency difference being zero indicating no rotation at all. The lock-in characteristic arises due to mutual coupling between the oppositely traveling waves. The dominant source of the coupling is mutual scattering of energy from each of the beams into the direction of the other. The effect is similar to lock-in coupling effects which have been long understood in conventional electronic oscillators.
Of course, any inability to accurately measure low rotation rates reduces the effectiveness of a laser angular rate sensor in navigational systems. Thus, much developmental work has been conducted in the field of laser angular rate sensors for purposes of reducing or eliminating the effects of "lock-in" so that the laser angular rate sensor may be more effectively used in navigational systems. A major advancement in this area was disclosed in U.S. Pat. No. 3,373,650, wherein a biasing system was provided which introduced a varying bias in the frequency of at least one of the counter-traveling beams of electromagnetic energy causing a varying frequency difference between the oppositely traveling beams of electromagnetic radiation, the bias being such that the varying frequency difference alternated in sign. The frequency bias so provided is such that there exists a frequency difference between the two oppositely traveling beams which is greater than the frequency difference which occurs near the lock-in rate for a majority of time. The sign or polarity of the frequency difference is alternated, typically periodically, so that the time integrated frequency difference between the two beams integrated over the time interval between sign reversals reversing from the same sign direction is substantially zero. Note that at those instances of time when the sign or direction of the frequency difference reverses, the two beams will tend to lock-in since at some point the frequency difference there between is zero. Since the gyro output angle is generally derived from the frequency difference which locks in to zero, even though there exists some rotation, an error accumulates in the gyro output angle. The periods of time when the two beams are "locked-in" usually are very short time intervals, and any possibly resulting gyro output angle error resulting therefrom is greatly reduced. Nevertheless, the error resulting from these periods of time during lock-in corresponding to each sign reversal of the frequency difference accumulate in the gyro output angle signal, and in time can amount to a bothersome level, particularly in precision navigational systems. This error is sometimes referred to as random walk or random drift.
The bias provided in such biasing systems as disclosed in U.S. Pat. No. 3,373,650, is sometimes referred to as dither, and a ring laser gyro having such dither is referred to as a dithered gyro. Hereafter, a dithered gyro is one in which a bias is introduced into the frequencies of the counter-traveling beams whereby the frequency difference between the beams varies with time and alternates in sign. The alternation in sign need not be periodic in nature, i.e. not perfectly repetitious. The bias introduced may be provided by inertial rotation of the gyro (mechanical dithering) or may be provided by directly affecting the counter-traveling beams (electrical or optical dithering).
Many improvements have been made to the basic dithered gyro disclosed in U.S. Pat. No. 3,373,650. One such improvement is disclosed in U.S. Pat. No. 3,467,472 wherein the improvement consists of randomly changing the amount of bias introduced into the counter-traveling beams in order to reduce the random walk resulting from those time intervals when the frequency of the beams are locked-in.
Heretofore, prior art dithered ring laser gyros do not determine the contribution of lock-in and other such light scattering errors which are included in the gyro output angle so that either compensation can be provided, or additional gyro control can be employed to reduce the error included in the gyro output angle derived from information of the two counter-traveling beams therein.